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This invention is related to a novel process for treatment of water in operations for the recovery of oil from geological formations. More specifically, this invention relates to novel, improved techniques for the preparation of high quality water for down-hole use, and more specifically, for generation of steam for downhole use, in heavy oil recovery operations.
Water treatment is a necessary operation in heavy oil recovery operations. This is because in order to recover heavy oil from certain geologic formations, steam is required to increase the mobility of the oil in the formation. Traditionally, heavy oil recovery operations have utilized xe2x80x9conce throughxe2x80x9d type steam generators. Such steam generators are most commonly provided in a configuration and with process parameters wherein about eighty percent (80%) quality steam is produced at about 1000 pounds per square inch (psig). The 80% quality steam (which is 80% vapor, 20% liquid, by weight) is injected via injection wells to fluidize the heavy oil. An oil/water mixture results, and the mixture is pumped to the surface. Then, the sought-after oil is separated from the water and recovered for sale.
The produced water stream, after separation from the oil, is further de-oiled, and is treated for reuse. Most commonly, the water is sent to the xe2x80x9conce-throughxe2x80x9d steam generators for creation of more steam for oil recovery operations. The produced water stream is typically required to have less than about 8000 PPM TDS (as well as meeting other specific constituent requirements) for re-use. Thus, in most cases, the recovered water must be treated before it is sent to the steam generators. Normally, such treatment is initially accomplished by using a warm lime softener, which removes hardness, and which removes some silica. Then, an xe2x80x9cafter-filterxe2x80x9d is often utilized, to prevent carry-over of any precipitate or other suspended solids. For polishing, in a hardness removal step, a weak acid cation (WAC) system is often utilized to simultaneously remove hardness and the alkalinity associated with the hardness. A schematic of such a typical prior art process is presented in FIG. 1.
A relatively new heavy oil recovery process, referred to as the Steam Assisted Gravity Drainage heavy oil recovery process (the xe2x80x9cSAGDxe2x80x9d process), ideally utilizes 100% quality steam for injection into wells (i.e., no liquid water). Initially, water utilized for generating steam in such operations can be treated much the same as in the just discussed traditional heavy oil recovery operations. However, in order to produce 100% quality steam using a once-through type steam generator, a series of vapor-liquid separators are required to separate the liquid water from the steam. The 100% quality steam is then sent down the well and injected into the desired formation. A typical process flow sheet for such a process is presented in FIG. 2.
Another method for generating the required 100% quality steam involves the use of packaged boilers. Various methods are well known for producing water of sufficient water to be utilized in a packaged boiler. One method which has been developed for use in heavy oil recovery operations involves de-oiling of the produced water, followed by a series of physical-chemical treatment steps. Such additional treatment steps normally include such unit operations as warm lime softening, after-filtration, organic traps, pre-coat filters or ultrafiltration, reverse osmosis, and mixed bed demineralization. A typical state-of-the-art process flow scheme for utilizing packaged boilers in heavy oil recovery operations is shown in FIG. 3. Such a physical-chemical treatment system may have a high initial capital cost, and generally involves significant ongoing chemical costs. Moreover, there are many waste streams to discharge, involving a high sludge disposal cost. Further, where membrane systems such as ultrafiltration or reverse osmosis are utilized, relatively frequent membrane replacement is encountered, at significant additional cost. Also, such processes can be quite labor intensive to operate and to maintain. Therefore, it is clear that the development of a simpler, more cost effective approach to produced water treatment as necessary for packaged boiler make-up water would be desirable.
In summary, the currently known and utilized methods for treating heavy oil field produced waters in order to generate high quality steam for down-hole are not entirely satisfactory because:
most physical chemical treatment systems are quite extensive, are relatively difficult to maintain, and require significant operator attention;
they often require liquid-vapor separation equipment, which adds to equipment costs;
a large quantity of unusable hot water is created, and the energy from such water must be recovered, as well as the water itself, in order to maintain an economic heat and material balance in plant operations;
they require large amounts of expensive chemicals, many of which require special attention for safe handling, and which present safety hazards if mishandled;
the treatment train produces fairly substantial quantities of undesirable sludges and other waste streams;
the disposal of waste sludges and other waste streams is increasingly difficult, due to stringent environmental and regulatory requirements.
Thus, it can be appreciated that it would be advantageous to provide a new process which minimizes the production of undesirable waste streams, while minimizing the overall costs of owning and operating a heavy oil recovery plant.
My novel water treatment process can be advantageously applied to heavy oil production operations. The method is particularly advantageous in that is minimizes the generation of waste products, and is otherwise superior to water treatment processes heretofore used or proposed.
From the foregoing, it will be apparent to the reader that one important and primary object of the present invention resides in the provision of a novel process for the treatment of produced waters, so that such waters can be re-used in heavy oil recovery operations.
Another important objective of my process is to simplify the processing procedures, which importantly, simplifies operations and improves quality control in the manufacture of high purity water for down-hole applications.
Other important but more specific objects of the invention reside in the provision of an improved water treatment process for production of high purity water for down-hole use, as described herein, which:
eliminates the requirement for separation of hot liquids from generated steam;
eliminates the generation of softener sludges;
in conjunction with the just mentioned object, minimizes the production of undesirable waste streams;
minimizes operation and maintenance labor requirements;
minimizes maintenance materiel requirements;
reduces the capital costs for water treatment equipment in heavy oil operations;
virtually eliminates chemical additives and associated handling requirements.
Other important objects, features, and additional advantages of my invention will become apparent to the reader from the foregoing and from the appended claims and the ensuing detailed description, as the discussion below proceeds in conjunction with examination of the accompanying drawing.
I have now invented, and disclose herein, an improved water treatment process for heavy oil recovery operations. This process minimizes or eliminates waste streams which must be sent for off-site disposal of liquids or sludges, and otherwise does not have the above-discussed drawbacks common to heretofore-utilized water treatment techniques of which I am aware. Importantly, my novel process overcomes the heretofore-encountered shortcomings of water handling in heavy oil recovery operations. Also, it eliminates undesirable multiplicity of treatment steps and unit operations seen in the more commonly utilized alternative water treatment processes. Thus, it is believed that my novel water treatment process will substantially simplify water treatment preparatory to steam generation in heavy oil recovery operations. My method involves producing steam from a selected feedwater, by first removing oil and grease to desired level, preferably to about twenty parts per million, or less. Then, the pH is adjusted, normally downward and by acid addition, to release at least some carbonate alkalinity as free carbon dioxide. Preferably, all non-hydroxide alkalinity is removed, or substantially so, by introducing the feedwater into a decarbonator, wherein said free carbon dioxide is substantially removed. Next the feedwater is introduced into an evaporator, and the feedwater is evaporated to a pre-selected concentration factor to produce (1) a distillate having a small amount of residual hardness therein, and (2) a concentrate containing residual solids (dissolved solids and in some cases, suspended solids). The distillate is then cooled, and is subsequently introduced into a hardness removal step, preferably an electrodeionization (EDI) treatment unit, to remove the residual hardness from the distillate. Finally, the substantially hardness free treated water stream is sent to packaged boilers, for production of high quality steam.